Prior-art workers have used microfibers to create superior acoustic and thermal insulating webs, taking advantage of insulating effects associated with the large surface area of the fine-diameter microfibers. Staple fibers have been blended with the microfibers in this prior work to open the web, thereby increasing the effectiveness of the microfibers and improving the insulating properties of the web (see, for example, U.S. Pat. Nos. 4,118,531 and 5,298,694). The prior-art microfiber-based insulating webs have developed important commercial acceptance and value; but improvement is continually sought, and the present invention makes possible an advance in these webs—e.g., an improvement in insulating properties—as discussed below.
The present invention is also an advance in another nonwoven web technology, which was first developed many years ago, even before development of the just-described insulating webs (see U.S. Pat. Nos. 3,607,588; 3,676,239; 3,738,884; 3,740,302; 3,819,452; and U.K. Patent No. 1,190,639, all issued from a line of patent applications originally filed in 1966). This technology involved the collection of spray-spun filamentary material with a collector consisting of two spaced-apart, contrarotating rolls disposed in the path of the material issuing from the extrusion orifice. The gap between the rolls was substantial, and only portions of the spray-spun filamentary material were deposited directly on the roll surfaces. The remainder of the filamentary material crossed back and forth randomly between the layers of material deposited on the roll surfaces to form a bridging structure connecting the layers together.
An object of this prior-art development was to provide nonwoven fibrous structures in which each of the opposed surfaces of the web consists of a densified layer, with those densified surface layers being connected by an integrally formed core made up of fibrous components bridging the space between the surface layers. A particular use of the technique was to provide pile-like fabrics formed by splitting the collected web lengthwise between and parallel to the surface layers. The dense surface layers, which desirably were collected on smooth-surfaced solid (nonporous) rolls while the fibers were tacky, served as a backing for the fabric, and the cut bridging structure between the surface layers became the “pile,” or upstanding fiber portion. In a representative example, the fibers had a diameter of about 24 micrometers.
When observed in a longitudinal vertical cross-section through the described collected web, the fibers exhibited a C-shaped configuration. A segment (or segments) of a representative individual fiber was disposed so as to be generally transverse or perpendicular to the faces of the web (this segment(s) formed the vertical portion of the “C”), and other segments of the fiber connected to the transverse segment(s) lay within the faces of the web (the arms of the “C”). Also, the C shapes were discrete from one another. That is, the fibers were grouped into sheets or subassemblies, each of which had a C-shaped configuration. The discrete C-shaped sheets or subassemblies were spaced apart in the machine direction of the web. That is, the arms of adjacent C-shaped subassemblies overlapped and formed the faces of the web, but the transverse portion of the C's were spaced apart, thus leaving large channels or voids within the collected webs that occupied almost the full height of the web and appeared to extend across the width of the web.
Another prior-art use of fibers in a C-shaped configuration is found in a series of patents issued in the U.S. in 1983-84 (U.S. Pat. Nos. 4,375,446; 4,409,282; and 4,434,205), based on original filings in Japan in 1978-79. These patents teach the collection of meltblown fibers in the “valley-shaped” zone between two separated porous plates or rollers. The collected webs are rather compact (one of the plates is often referred to as a presser plate, though it is stated that compression is not always necessary). A preferred use for the collected webs seems to be as synthetic leather; other described uses are electrical insulators, battery separators, filters, and carpets.
A more recent patent publication, WO 00/66824, published November 2000, also teaches webs with fibers collected in C-shaped configuration. The collected fibers are said to be folded to form loops, with the loops forming “a train of waves spaced along the machine direction, running from edge to edge in the cross direction and extending in the z-direction” (through the thickness of the web). Large channels or voids are pictured running through the width of the web. Either meltspun or meltblown webs are contemplated, and the meltblown webs may be a “coform” type of web; the latter are described with reference to U.S. Pat. No. 4,818,464 as containing other materials such as pulp, superabsorbent particles, cellulose or staple fibers, exemplified as cotton, flax, silk or jute.
The densified, compacted, or channeled webs of the prior art may be adapted to particular uses as described in the patents, though we are unaware of any commercial products that have resulted from these prior-art teachings.